I'm considering semi-permanently mounting an AT822 stereo mic in a local venue. I probably do a soundboard/aud matrix recording 2-3 times a month there and it's impossible to get good mic placement unless I set mics up on the small stage but that's not always feasible. So for now I set mics up off to the far side of the room and have had decent results.

So it it safe (or crazy) for me to assume that if I have the mounted mic turned on and change the battery once a month, I'll be ok? Doing the math I should get 40 or so days of life out of a battery but I surely don't want to push my luck. And I also don't want to have to get up there and deal with it everytime I record.

I would use a Lithium AA battery. Energizer specifies a nominal capacity of about 3500...3600mAh (here: http://data.energizer.com/pdfs/l91.pdf ). Audio Technica lists the current drain at 1.2mA for the AT822, so you can expect a battery lifetime of about 4 months. Changing batteries three times a year should be no problem.

Another advantage is the sightly higher voltage of 1.7V for the lithium cell, giving more headroom for the mic capsule.

I would use a Lithium AA battery. Energizer specifies a nominal capacity of about 3500...3600mAh (here: http://data.energizer.com/pdfs/l91.pdf ). Audio Technica lists the current drain at 1.2mA for the AT822, so you can expect a battery lifetime of about 4 months. Changing batteries three times a year should be no problem.

Another advantage is the sightly higher voltage of 1.7V for the lithium cell, giving more headroom for the mic capsule.